The application of thin films in optical interference coatings has been well established in optical technology for many years. The purpose of these vacuum deposited films was to provide thin films with accurately reproducible refractive indices and to control thicknesses. However, the main emphasis to date has been directed towards achieving films of uniform refractive indices. The primary reason for this is that all multi-layer films have been calculated on the use of homogeneous films, i.e., films with a constant index of refraction through each individual layer, and films which deviate from this homogeneity generate results which are almost impossible to analyze in detail, details which cause the desired multi-layered design to fail. If inhomogeneities are present in any one film having multi-layers, reproduction consistency is almost impossible to maintain.
It is the current practice of persons skilled in the art of large area solar energy transmissive/reflective coatings to utilize thin film coatings of one or more homogeneous layers, single layer coatings of aluminum based films predominating. Deposition is typically accomplished by vacuum evaporation of the film material onto a dynamic substrate. Such practice, however, has severe limitations. First, evaporation techniques are constrained by the vaporization temperature of the film material. As there is an effective upper limit of temperature at which vaporization can be employed, the practitioner is limited in the choice or selection of materials which can be used. Further, homogeneous thin films are characterized by spectral reflective properties occurring at the interface of dissimilar homogeneous layers. What results is the creation of an overall mirror-like appearance that detracts from use of such coatings in residential units and the like. While these spectral reflective characteristics can be minimized by reducing the refractive index of the film to a value closer to that of the substrate, this is achieved at the expense of an overall loss in the efficiency of the coating to screen out undesired heat producing wavelengths of radiant energy, i.e., infra-red radiation.
The concept of deliberately utilizing a varying index of refraction through a continuously prescribed formula has stimulated optical research over the last few years. See, for example, R. Jacobsson, "Inhomogeneous and Coevaporated Homogeneous Films for Optical Applications," Physics of Thin Films, Volume 8, pp. 51-98 (G. Hass, M. Francombe, R. Hoffman ed. 1975). Such an achievement would result in the design and production of an interference coating with optical properties superior to those of conventional multi-layer homogeneous thin films. However, recent attempts at the experimental attainment and control of inhomogeneous films have met with only limited success.
A promising method of inhomogeneous thin film construction involves the use of sputtering techniques to deposit select materials and combinations of said materials with reactive gases upon a substrate. A film layer deposited in the presence of an inert atmosphere comprising, for example, a predetermined partial pressure of argon results in the deposition upon the substrate of pure magnetron cathode material. By introducing a reactive gas, such as oxygen, during the sputtering process, i.e., reactive sputtering, said material chemically combines with the gas and produces the deposition of a material having a refractive index different from that of the cathode material. Control of reactive gas flow rate into the magnetron chamber permits the controlled variation of the refractive index throughout the film depth. See, for example, S. Ingrey, U.S. Pat. No. 3,962,062, disclosing a method for depositing an inhomogeneous film upon a stationary substrate by controllably varying the ratio of the flow rate of O.sub.2 in relation to the flow rate of N.sub.2 during the sputtering process of tantalum or niobium.
While recognizing that inhomogeneous thin films possess optical properties rendering them superior in particular applications to conventional homogeneous films, applicant is not aware of any method which has been devised to date whereby inhomogeneous films may be controllably deposited upon large surface area substrates.